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1. Technical Field of the Invention
This invention relates to a crystal-structured semiconductor film, semiconductor device using the same and method for manufacturing those, and more particularly to a semiconductor film excellent in crystallinity and having a crystal orientation arranged in a single direction, semiconductor device using the same and method for manufacturing those.
2. Description of the Related Art
There is a technology, called a laser anneal process, developed as a method to crystallize an amorphous silicon film formed on an insulating substrate of glass or the like. In the laser anneal process, a laser light having an energy of approximately 100-500 mJ/cm2 is radiated to an amorphous silicon film, thereby realizing crystallization.
For amorphous silicon crystallization, there is a need to heat it up to usually 600xc2x0 C. or higher. The laser anneal process, however, has an extremely excellent feature that can crystallize an amorphous silicon film while keeping a substrate at nearly a room temperature. The laser uses a solid laser, as represented by an excimer laser or a YAG laser. In any way, because of limitation in beam size, the processing of a large-area substrate requires radiation by connection with bean scans. Accordingly, there is a disadvantage pointed out that crystallinity changes at connections thus disabling to obtain a uniform crystal. Meanwhile, in the case of laser anneal, there also is difficulty in obtaining a homogeneous crystal because of instable output of a laser oscillator. Such crystal quality variation is responsible for the characteristic variation in thin film transistors (hereinafter, described as TFTs).
On the other hand, Japanese Patent Laid-Open No. 7-231100, Japanese Patent Laid-Open No. 7-130652, Japanese Patent Laid-Open No. 8-78329, etc. disclose an art that, using a catalyst element for accelerating the crystallization of an amorphous silicon film, a heating process is made at a temperature of 450-650xc2x0 C. to cause crystallization in a part or the entire of an amorphous silicon film, and heating is further made at a temperature higher than that heating temperature to thereby obtain a large-grained crystalline silicon film.
In order to obtain a high-quality crystalline silicon film, it is emphasized to arrange the orientation of crystals besides the increase of crystal-grain size. It is considered, in the laser anneal process, that crystallization proceeds on the basis of the spontaneous nucleation of crystals at the interface between an amorphous silicon film and a substrate. The silicon film crystallized by this method, when analyzed in the crystal structure by X-ray diffraction, is usually observed with diffraction peaks at (111), (220), (311) and so on. It has been confirmed as a polycrystalline body aggregated with various orientations. In the polycrystalline body, individual crystal grains precipitate on arbitrary crystal planes. In this case, the probability is the greatest that crystal precipitation occurs on a (111) plane where the interface energy is minimized to an underlying silicon oxide.
In the case that a catalyst element for accelerating silicon crystallization is introduced into an amorphous silicon film to cause crystallization, formed is a silicide of an element introduced at a temperature lower than a temperature of spontaneous nucleation, causing crystal growth on the basis of the silicide. For example, NiSi2 under forming does not have a particular orientation. However, in case the amorphous semiconductor film is reduced to a thickness of 200 nm or less, growth is allowed substantially only in a direction parallel with a substrate surface. In this case, minimum is the interface energy at the contact between the NiSi2 and the crystal-silicon (111) plane. Thus, the plane parallel with a crystalline silicon film surface is a (110) plane, in a lattice plane of which preferential orientation made. Where the crystal grows in a columnar form in a direction parallel with the substrate surface, there exists a freedom in a direction about an axis of the columnar crystal. Thus, orientation is not always on the (110) plane and precipitation occurs also on the other lattice planes. The percentage of orientation on the (110) plane is, however, still less than 20 percent in total.
In the case of low orientation ratio, it is almost impossible to maintain a lattice continuity at a crystal boundary where crystals with different orientations crash one against another. Easily presumed is formation of a number of dangling bonds. The dangling bond at a grain boundary acts as a recombination center or trap center, to reduce the property of carrier (electron/hole) transport. As a result, there is a problem that, because the carriers are vanished due to recombination or trapped in defects, high mobility is not to be expected by the use of such a crystalline semiconductor film.
There is a disclosure, in Japanese Patent Laid-Open No. 2000-114172, of an art that crystallization is made by adding a proper amount of germanium to a silicon film in order to enhance crystal orientation ratio. This publication indicates to obtain a semiconductor film that can be substantially considered as a single crystal exhibiting such a crystallinity that individual crystal grains are arranged in plane orientation order despite it is a semiconductor film aggregated with a plurality of crystal grains. In obtaining it, however, a thermal process at 900-1200xc2x0 C. is required besides the addition of germanium.
In this manner, crystal quality can be improved by carrying out a thermal process at a high temperature exceeding 900xc2x0 C. However, such a thermal process cannot be carried out for a crystalline silicon film formed on a glass substrate less resistive to heat. Also, there is a problem that, even if the orientation ratio is enhanced by germanium addition, germanium low in combination energy with hydrogen is not easy for hydrogenation. Namely, a hydrogenation process cannot compensate for the dangling bond caused by germanium.
It is an object of the present invention to provide means for solving the foregoing problem, and to enhance the orientation ratio of a crystalline semiconductor film obtained by crystallizing an amorphous semiconductor film while using as a substrate a less heat-resistive material such as glass thereby providing a semiconductor device using a crystalline semiconductor film with the high quality equivalent to a single crystal.
In order to solve the foregoing problem, in the present invention, a first crystalline semiconductor film is formed containing first and second elements and having a high crystal orientation, relying upon a crystal orientation of which is formed a second crystalline semiconductor layer based on the first element and having a high orientation ratio. The second element is used to improve orientation ratio. In order to obtain a high-quality crystalline semiconductor film and semiconductor device using same, it is satisfactory to substantially use a crystalline semiconductor film based only on the first element. In view of this point, the present invention has the following structure.
The present invention crystallizes a first amorphous semiconductor film formed on a substrate having an insulating surface, and then deposits thereon a second amorphous semiconductor film and crystallizes it. The second amorphous semiconductor film is epitaxially crystallized relying upon a crystal of an underlying first crystalline semiconductor film.
Accordingly, the crystallinity of the first crystalline semiconductor film is an important characteristic parameter. The means for enhancing the orientation of the first crystalline semiconductor film includes the application with an amorphous semiconductor film containing germanium in a ratio of 0.1 to 10 atom percent to silicon, and a catalyst element acting to accelerate the crystallization in the amorphous semiconductor film.
The element for accelerating crystallization (catalyst element) uses one or a plurality of those selected from Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu and Au. Meanwhile, the amorphous semiconductor film is formed in a thickness of 10 nm to 200 nm. By adding the metal element to the amorphous silicon film and carrying out a heating process, formed is a compound of silicon and the metal element (silicide). This is diffused to proceed with crystallization. The germanium added in the amorphous silicon film, not reacting with this compound, exists at an around thereof to cause local strain. The strain acts toward increasing a critical radius of nucleation, to reduce nucleation density and possess an effect to restrict crystal orientation.
It has been found, as a result of experiment, that the concentration of germanium required for causing such an action may be 0.1 atom percent or more and 10 atom percent or less, preferably 1 atom percent or more and 5 atom percent or less to silicon. In case the concentration of germanium exceeds the upper limit value, spontaneous nucleation (nucleation regardless of a compound with an added metal element) is prominent occurring as an alloy material of silicon and germanium, making impossible to enhance the orientation ratio in an obtained polycrystalline semiconductor film. Meanwhile, in the case of below the lower limit value, sufficient strain cannot be caused, also making impossible to enhance the orientation ratio.
The amorphous silicon film added with germanium is formed by a plasma CVD process using intermittent or pulse discharge. The intermittent or pulse discharge is formed by modulating a radio power having an oscillation frequency of 1-120 MHz, preferably 13.56-60 MHz, into a repetitive frequency of 10 Hz-10 kHz and supplying it to a cathode. The ratio in time of radio power application in one period of the repetitive frequency, if given as a duty ratio, is provided with a value of 1-50 percent.
With such an intermittent or pulse discharge, selected is a radical species in a deposition process of an amorphous semiconductor film (herein, refers to an electrically neutral, chemically active atom or molecule) so that film growth can be made with a comparatively long-life radical species. For example, when discomposing SiH4 in a discharge space, various radical or ion species are caused. The radical species repeats the reactions of generations and vanishments. In a steadily sustained discharge, the radical species are kept at a constant existence ratio. However, in the case that there exists a time of off-discharge as in intermittent or pulse discharge, only the long-life radical species is supplied onto a film deposition surface and contributes to film deposition, due to a difference in lifetime between radical or ion species.
The reason of selecting a long-life radical is to inactivate a film growth surface. Germanium is suited for being dispersed and contained in an amorphous silicon film. Because GeH4, a source of germanium, is low in decomposition energy as compared to SiH4, if decomposed at the same supply power, causes atomic germanium, to cause germanium clusters due to gas-phase or surface reaction. According to the foregoing crystal growth model, because germanium is preferably dispersed, desired is intermittent discharge not to cause clusters.
When an amorphous semiconductor film is crystallized, the film volume reduces due to atom rearrangement. As a result, the polycrystalline semiconductor film formed on a substrate involves tensile stresses. However, by containing germanium having a greater atomic radius in a range of 0.1 atom percent or more and 10 atom percent or less, preferably 1 atom percent or more and 5 atoms percent or less, in silicon, the volume contraction upon crystallization is suppressed to reduce the internal stress caused. In this case, in order to obtain a homogeneous effect throughout the film, germanium preferably exists in a dispersed state.
However, germanium has a great atomic radius as compared to silicon. This, when included in silicon, causes a factor to strain the crystal. Meanwhile, because germanium is difficult to compensate for the defects due to hydrogenation, the concentration thereof after crystallization is desirably reduced to a possible low extent. Specifically, utilized is the phenomenon of germanium segregation upon fusion-solidification of a semiconductor containing silicon and germanium. Such a semiconductor film can be easily fused-solidified by laser radiation. The high concentration germanium region segregated with germanium may be removed by chemical etching or chemical mechanical polish to reduce the thickness of a first crystalline semiconductor film. It is preferred that the first crystalline semiconductor film, in its surface, is treated with a solution containing hydrogen fluoride to form a clean surface, and then a second amorphous semiconductor film is deposited thereon. However, air-constituent elements, such as absorbed oxygen, carbon, and nitrogen, may somewhat remain on the surface.
In this manner, a second amorphous semiconductor film is formed on the first crystalline semiconductor film having a high orientation ratio, and crystallized by a heating process, such as furnace anneal or rapid thermal anneal (RTA) or laser radiation. The crystal is allowed to grow in the same plane orientation relying upon the underlying crystal orientation.
As in the above, a method for manufacturing a semiconductor device according to the present invention comprises the steps of: forming a first amorphous semiconductor film containing germanium in a ratio of 0.1 to 10 atom percent to silicon; adding an element having a catalytic action for crystallization to the first amorphous semiconductor film; thereafter carrying out a first crystallizing process with a heating process in an inert gas and a second crystallizing process with radiation of a laser light in an oxidizing atmosphere, to form a first crystalline semiconductor film; removing the first crystalline semiconductor film by a predetermined thickness from its surface; thereafter forming a second amorphous semiconductor film based on silicon on the first crystalline semiconductor film; and crystallizing the second amorphous semiconductor film in an inert gas to form a second crystalline semiconductor film.
Also, another structure comprises the steps of: forming a first amorphous semiconductor film containing germanium in a ratio of 0.1 to 10 atom percent to silicon; adding an element having a catalytic action for crystallization to the first amorphous semiconductor film; thereafter carrying out a first crystallizing process with a heating process in an inert gas and a second crystallizing process with radiation of a laser light in an oxidizing atmosphere, to form a first crystalline semiconductor film; removing the first crystalline semiconductor film by a predetermined thickness from its surface; repeating a plurality of number of times the first crystallization process, the second crystallization process and the etching process in the order; thereafter forming a second amorphous semiconductor film based on silicon on the first crystalline semiconductor film; and crystallizing the second amorphous semiconductor film in an inert gas to form a second crystalline semiconductor film.
The means for removing the first crystalline semiconductor film by a predetermined thickness may be applied with any of wet etching, dry etching and chemical mechanical polish. In the case of using wet etching, it can be made with an etching solution containing HNO3, HF, CH3COOH and Br2, or an etching solution containing HNO3, HF, CH3COOH and I2.
Meanwhile, the catalyst element used in crystallizing the first amorphous semiconductor film is removed by gettering. The gettering may be carried out after the second crystallizing process or after forming a second crystalline semiconductor film.
The obtained crystalline semiconductor layer has: a second crystalline semiconductor film based on silicon provided in close contact with a first crystalline semiconductor film containing silicon and germanium; wherein the first crystalline semiconductor film has a (101)-plane orientation ratio of 30 percent or greater and the second crystalline semiconductor film has a (101)-plane orientation ratio of 20 percent or greater. Meanwhile, the first crystalline semiconductor film contains germanium in a concentration of 1xc3x971020 /cm3 or less and the second crystalline semiconductor film contains germanium in a concentration of 1xc3x971019 /cm3 or less. Also, provided is a crystalline semiconductor layer that the first crystalline semiconductor film and the second crystalline semiconductor film are coincident in crystal orientation at a ratio of 60 percent or higher.
Meanwhile, the invention forms a first crystalline semiconductor film having a high orientation ratio on a substrate, on which an amorphous silicon film is formed as a second semiconductor layer. By carrying out a laser radiation for crystallization, a semiconductor layer having a high orientation ratio is obtained under the influence of the high orientation ratio of the first crystalline semiconductor layer. Particularly, the first semiconductor layer suitably uses a silicon-germanium (Si1-xGex) film.
A Si1-xGex film having a high orientation in the same plane direction is obtained by adding a catalyst element to a Si1-xGex (x=0.001-0.05) film formed by a plasma CVD process and carrying out a heating process on it. The first crystalline semiconductor layer (crystalline Si1-xGex film), obtained by catalyst element addition and heating process, has a high (110)-plane orientation.
Then, an amorphous silicon film is formed as a second semiconductor layer on the first crystalline semiconductor layer, and laser light is radiated to it. At this time, the orientation of the first crystalline semiconductor layer has an effect upon the crystal orientation of the second semiconductor layer (amorphous silicon film), to obtain a crystalline silicon film having a high first (110)-plane orientation. By using the first crystalline semiconductor layer as a seed (nucleus) in a crystallization process for the second semiconductor layer, it is possible to form a preferred crystalline semiconductor layer having a high orientation ratio.
Subsequently, because the catalyst element used in forming the first crystalline semiconductor layer and remaining in the semiconductor layer possibly has a bad effect upon the characteristic of a TFT made using the semiconductor layer, carried out is a process for moving the catalyst element from the semiconductor layer.
A gettering region is formed on the second semiconductor layer. Incidentally, prior to forming a gettering region, a chemical oxide film may be used which is to be formed as a barrier layer on the second semiconductor layer by processing with an ozone-containing solution. On the barrier layer, a semiconductor layer as a gettering region is formed by a sputter or plasma CVD process. Incidentally, the gettering region, because to be removed later by etching, preferably uses a low density film, such as an amorphous silicon film, having a high selective ratio with respect to the crystalline semiconductor layer.
Subsequently, an inert gas element is added to the gettering region. The inert gas element may use one or a plurality of those selected from He, Ne, Ar, Kr and Xe. Incidentally, when forming a gettering-region semiconductor layer, the inert gas, if introduced into the semiconductor layer, can form a gettering region.
Next, carried out is a heating process for moving the catalyst element to the gettering region. The heating process may use any of a method of heating using the radiation heat of a light source, a method of heating with a heated inert gas and a method of heating using a furnace. By such a heating process for gettering, the catalyst element is moved into the gettering region thereby reducing the concentration of the catalyst element remaining in the semiconductor layer down to 1xc3x971017 /cm3 or less. After ending the gettering process, the gettering region is removed away.
In this manner, a first crystalline semiconductor layer having a high orientation ratio is formed and a second semiconductor layer is formed thereon, followed by radiating a laser light in order for crystallization. Due to this, the second semiconductor layer can also be made into a crystalline semiconductor layer having a high orientation ratio under the influence of the orientation of the first crystalline semiconductor layer.